A publicly known electric motor-driven booster is disclosed, for example, in Patent Literature 1. The electric motor-driven booster has an input rod connected to a brake pedal, a booster piston relatively movably fitted around the input rod, an electric motor for driving the booster piston, and a controller for controlling the operation of the electric motor according to the movement of the input rod. The input rod and the booster piston propel a piston in a master cylinder, and driving force of the electric motor is applied to obtain a desired boost ratio with respect to the operating force of the brake pedal.
In other words, the boost ratio can be varied by adjusting the relative positional relationship between the input rod and the booster piston, and thus it is possible to execute various brake control operations, such as boost control, brake assist control, regenerative cooperative control, and so forth. In the event of a failure in the electric motor or the like, the input rod directly operates the booster piston, thereby enabling the braking function to be maintained.